Prior art fully skirted hover-craft, such as disclosed in the patent to Leonard, U.S. Pat. No. 3,850,126 use fore and aft air cushion plenum subdivision, achieved by a transverse skirt or seal barrier located in the vicinity of the vehicle's center of gravity to provide static pitch trim stability when a load is carried on a deck on the forward part of the craft. The static stabilization in the loaded condition is accomplished by maintaining an over-pressure in the forward compartment relative to the aft compartment. Both compartments have approximately the same cushion pressure in the unloaded condition. An air passage duct provides a path for transferring air to and from the fore and aft cushion compartments. A flow control valve mounted in the duct, may be manually or automatically adjusted to support loads of varying weight on the forward deck. This control means is intended primarily to regulate static trim according to the weight distribution.
The disadvantage of the foregoing type of stabilization as illustrated by the patent to Leonard is that it does not provide adequate air flow for dynamic pitch trim control of the vehicle in the unloaded or lightly loaded condition, but rather only static condition control, because there is little or no pressure differential from the aft compartment to the forward compartment. Also, no means is specified for automatically controlling the flow control valve in the duct for regulating ride quality in waves. In fact, Leonard suggests a higher pressure being maintained in the forward compartment. Thus his flexible transverse barrier would have to be of the closed finger type, that is, not open in the rear, because otherwise, an overpressure in the forward compartment would cause open fingers to collapse. Furthermore, these closed fingers are more complex and therefore more difficult to maintain.
In regard to rigid sidewall vehicles, such as the captured air bubble (CAB) vehicles, the prior art air cushion plenums are not subdivided transversely and achieve inadequate pitch trim stabilization from the bow and stern seals and the sidewalls. In waves, vehicle motion in pitch can be excessive causing wave impacts of great force which at least make for an uncomfortable ride and can be damaging.